1. Field of the Invention
The present invention relates generally to multilayer circuit assemblies, and more particularly to pad shapes for improved etching of tri-metal-layered multilayer circuit assemblies.
2. Disclosure Information
U.S. Pat. No. 3,801,388 to Akiyama et al. (hereinafter "Akiyama"), U.S. Pat. No. 4,404,059 to Livshits et al. (hereinafter "Livshits"), and U.S. Pat. No. 5,738,797 to Belke, Jr. et al. (hereinafter "Belke"), all of which are incorporated herein by reference, disclose various methods for making electronic circuits which feature circuit crossovers or "air bridges" using various combinations of plating, masking, and etching steps. The methods disclosed in these references are useful for making multilayer circuits by selectively etching a tri-metal-layered precircuit such that the desired air bridge and circuit layout structures are created.
For example, the tri-metal precircuit may comprise a structure similar to that illustrated in FIGS. 1-3. Here, the middle layer 10 is a continuous 6-mil-thick aluminum sheet or foil having a top surface 14 and a bottom surface 12, with a first (lower) conductor pattern 20 made of 2-mil-thick copper patterned onto the bottom surface 12, and a second (upper) conductor pattern 40 made of 2-mil-thick copper patterned onto the top surface 14. The lower conductor pattern 20 includes a plurality of base pads 26, pedestal pads 22, and circuit traces 24, while the upper conductor pattern 40 includes a plurality of top pads 41 and bridging elements 42. Each top pad 41 is arranged on the middle layer top surface 14 opposite a respective one of the base pads 26. Each bridging element 42 has first and second enlarged ends 44 and a constricted portion 46 between and contiguous with the enlarged ends. The bridging elements 42 are arranged on the top surface 14 such that each enlarged end 44 is disposed opposite a respective one of the pedestal pads 22 and each constricted portion 46 is disposed opposite and transverse to a respective one of the circuit traces 24.
This precircuit may then be affixed to a substrate 30 having an electrically insulative surface 32 by attaching the first conductor pattern 20 to this surface 32. Then, the precircuit may be exposed for a predetermined amount of time to an etchant (e.g., sodium nitrate) which etches substantially only the aluminum, resulting in the final circuit structure illustrated in FIGS. 4-6. Here, those portions of the aluminum foil 10 which are sandwiched between an enlarged end 44 and a pedestal pad 22, or between a top pad 41 and a base pad 26, are more protected from attack by the etchant than are other portions of the foil 10. After the predetermined amount of time has elapsed and the etching has ceased, most of the foil 10 has been etched away, except for a pedestal 48 of aluminum remaining sandwiched between (1) each matched pair of upper enlarged ends 44 and lower pedestal pads 22, and between (2) each matched pair of upper top pads 41 and lower base pads 26. This provides a plurality of air bridges 42 and a plurality of "towers" 43, as shown in FIGS. 4-6.
The air bridges 42 created by this process serve as three-dimensional crossovers. For example, signal or current may flow from point A to point B along the lower conductor pattern, then rise to point C through an air bridge pedestal, then flow across the air bridge to point D, then down the other pedestal to point E, and then on across the lower conductor pattern to point F, thus allowing the circuit trace path ABCDEF to "cross over" the circuit trace path between points G and H.
Each tower 43 comprises a top pad 41 atop a pedestal 48 atop a base pad 26, as shown in FIG. 6. These towers 43 may be sized and arranged to serve in a variety of interconnect configurations. For example, a given set of towers 43 may serve as wirebond pads, solder joint pads (e.g., for reflowed chip components), direct chip attachment, and the like. A circuit trace 24 is typically attached to the base pad 26 of each tower element 43.
The conductor patterns 20/40 may be formed on the aluminum sheet 10 by various methods disclosed in Belke, Livshits, and Akiyama. These references teach that the enlarged ends 44 and/or top pads 41 should be made a certain minimum size (e.g., 40 mils in diameter or smallest width, for the 2/6/2-mil example above) and the constricted portion 46 made a certain maximum size (e.g., no more than 5 mils wide), so that the foil 10 sandwiched between each pair of enlarged ends 44 and pedestal pads 22 and each pair of top pads 41 and base pads 26 is only partly etched through in the X direction leaving the desired pedestals 48, while all other portions of the foil 10 (including those portions underneath the constricted portions 46) are completely etched away.
Whenever there is ample circuit space available around a given air bridge or tower, one may design the enlarged ends 44 and/or top pads 41 well above the recommended minimum size, thus assuring the formation of sturdy pedestals 48 and a robust metallurgical connection between each pedestal and its upper enlarged end/top pad 44/41. However, in fine-pitch applications, or where circuit layout is particularly crowded, it may not be possible to design the ends/pads 44/41 oversized; in fact, when the size of the ends/pads 44/41 is kept close to the recommended minimum size, or even more so when it is desired to make these features even smaller than recommended, there is often a danger of the ends/pads 44/41 becoming delaminated from their respective pedestals 48 during etching, as illustrated in FIG. 7.
It would be desirable, therefore, to provide a way of keeping the size of the enlarged air bridge ends 44 and top pads 41 small, while minimizing the aforementioned risk of delamination.